Why were True and False changed to keywords in Python 3

Solution 1:

Possibly because Python 2.6 not only allowed True = False but also allowed you to say funny things like:

__builtin__.True = False

which would reset True to False for the entire process. It can lead to really funny things happening:

>>> import __builtin__
>>> __builtin__.True = False
>>> True
False
>>> False
False
>>> __builtin__.False = True
>>> True
False
>>> False
False

EDIT: As pointed out by Mike, the Python wiki also states the following under Core Language Changes:

Make True and False keywords.
- Reason: make assignment to them impossible.

Solution 2:

For two reasons, mainly:

So people don't do a __builtin__.True = False prank hidden on a random module. (as explayned by devnull)
Because keywords are faster than global builtins. In Python 2.x, the interpreter would have to resolve those variables' values before using them, which is a bit slower than keywords. (see Why is if True slower than if 1?)

Solution 3:

This was discussed some months ago on python-dev. Having tons of links to the definition of True would be annoying, contrary to links to e.g. the nonlocal or with statements doc.

And things I conclude why True and False will make things "even finer".

re-bound as a side effect of functions called within the loop.

It's really easy to change True, such as:

def True(): print True
There is really no good use case for letting user code rebind the built-in names None, True, and False, making them keywords has almost only pluses.

Make program has to look-up "True" in symbol-table at each step just to find True has value True is far from intuitive. (that is why 1 is faster than True.)

x=compile('while 1: foop()', '', 'exec')

dis.dis(x)

      0 SETUP_LOOP              19 (to 22)
      3 JUMP_FORWARD             4 (to 10)
      6 JUMP_IF_FALSE           11 (to 20)
      9 POP_TOP
>>   10 LOAD_NAME                0 (foop)
     13 CALL_FUNCTION            0
     16 POP_TOP
     17 JUMP_ABSOLUTE           10
>>   20 POP_TOP
     21 POP_BLOCK
>>   22 LOAD_CONST               1 (None)
     25 RETURN_VALUE

x=compile('while True: foop()', '', 'exec')

dis.dis(x)

          0 SETUP_LOOP              19 (to 22)
    >>    3 LOAD_NAME                0 (True)
          6 JUMP_IF_FALSE           11 (to 20)
          9 POP_TOP
         10 LOAD_NAME                1 (foop)
         13 CALL_FUNCTION            0
         16 POP_TOP
         17 JUMP_ABSOLUTE            3
    >>   20 POP_TOP
         21 POP_BLOCK
    >>   22 LOAD_CONST               0 (None)
         25 RETURN_VALUE

reference:

http://mail.python.org/pipermail/python-checkins/2011-September/107641.html
http://mail.python.org/pipermail/python-checkins/2012-January/110353.html
https://groups.google.com/forum/#!searchin/comp.lang.python/False$20True$20keyword/comp.lang.python/65B6qWUf8wI/HwQ5pwnW7woJ

Beginning talk related to assigning to True and False:

http://bugs.python.org/issue2349
https://groups.google.com/forum/comp.lang.python/65B6qWUf8wI/HwQ5pwnW7woJ

some Aux Data:

PS: some number shows True/1:

[alex@lancelot test]$ timeit.py -c -s'import itertools as it' 'c=it.count()'
'while True:' '  if c.next()>99: break'
10000 loops, best of 3: 91 usec per loop

[alex@lancelot test]$ timeit.py -c -s'import itertools as it' 'c=it.count()'
'while 1:' '  if c.next()>99: break'
10000 loops, best of 3: 76 usec per loop